Simon...At first look that leg does not look familiar, and either it is the wrong leg or the correct leg with a cable lug added. The weld does not look to a factory "doing it everyday" standard Got a feeling that for your year the leg has front and rear mudguard stay lugs and a brazed or welded on mudguard bridge/torque arm mounting.
That's all, and the cable goes straight to the brake backplate fulcrum pin, and on to the brake arm. Fit the arm so that when adjusted, and the brake applied, it approaches the magic 90 degrees, but does not pass over 90. The position of the squared cutouts on the arm can be changed by turning the arm over, to get the best compromise for brake adjustment.
Lots of plungers on the Forum, so you may even get a pic of a correct layout.
Whoops, BM got the answer while I was doing my post, so we both think that's the problem.
Swarfy.